The former Spice Girl, whose contestant Andrea Faustini is one of the three finalists battling it out, was absent from the press conference for the final this week, and now it's been revealed that she's in hospital for tests after collapsing at home.
An 'X Factor' spokesperson said: "Unfortunately Mel is still unwell and following doctors advice she will not be taking part in Saturday's show. Tulisa will join the panel as a guest judge."
Mel was last seen at the Royal Albert Hall on Tuesday, and was out touting for Andrea on the Battle Bus the same day. Now those close to her are reporting that her body's "gone into shutdown" due to extreme exhaustion.
A source close to the star told the Sun, "She is in a bad way. Everyone is very worried. We are all just praying she comes through this."
ITV have confirmeed Tulisa was chosen as Mel's replacement because of her previous involvement with the show. She also helped Louis Walsh choose his groups at this year's Judge's Houses stage of the competition.
The N-Dubz star also selected which song Stereo Kicks would be performing during Jukebox week.